Magnetic memory with phonon glass electron crystal material

ABSTRACT

A magnetic memory unit includes a tunneling barrier separating a free magnetic element and a reference magnetic element. A first phonon glass electron crystal layer is disposed on a side opposing the tunneling barrier of either the free magnetic element or the reference magnetic element. A second phonon glass electron crystal layer also be disposed on a side opposing the tunneling barrier of either the free magnetic element or the reference magnetic element to provide a Peltier effect on the free magnetic element and the reference magnetic element.

RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationNo. 61/104,078, filed on Nov. 18, 2008 and titled “Thermally AssistantMRAM and RRAM with Peltier Effect”. The entire disclosure of applicationNo. 61/104,078 is incorporated herein by reference.

BACKGROUND

Magnetic random access memory (MRAM) devices are solid state,non-volatile memory devices which make use of the giant magnetoresistiveeffect. A conventional MRAM device includes a column of first electricalwires, referred to as word lines, and a row of second electrical wires,referred to as bit lines. An array of magnetic memory cells, located atthe junctions of the word lines and bit lines, is used to record datasignals.

A magnetic memory cell includes a hard magnetic layer, a soft magneticlayer, and a non-magnetic layer sandwiched between the hard magneticlayer and the soft magnetic layer. The hard magnetic layer has amagnetization vector or orientation fixed in one direction. Theorientation of this magnetization vector does not change under amagnetic field or electron spin-torque applied thereon. The softmagnetic layer has an alterable magnetization vector or orientationunder a magnetic field applied thereon, that either points to the samedirection, hereinafter “parallel alignment”, or to the oppositedirection, hereinafter “antiparallel alignment”, of the magnetizationvector or orientation of the hard magnetic layer. Since the resistancesof the magnetic memory cell in the “parallel alignment” status and the“antiparallel alignment” status are different, the two types ofalignment status can be used to record the two logical states—the “0”sor “1”s of a data bit.

In a writing operation for one illustrative embodiment, an electriccurrent passes through the word line and the bit line adjacent to thememory cell. When the electric current reaches a certain threshold, amagnetic field generated by the electric current will switch theorientation of the magnetization vector of the soft magnetic layer. As aresult, the magnetization vector of the hard magnetic layer and the softmagnetic layer will be changed from one type of alignment, e.g.“parallel alignment”, to the other type of alignment, e.g. “antiparallelalignment”, so that a data signal in form of one data bit can berecorded in the memory cell.

In MRAM structure design, lower writing power dissipation and a highercell density are desired. Unfortunately, a reduction of cell size, i.e.an increase in cell density, leads to a reduction in the availableenergy (K_(u)V) to store the bit data. Further, the error rate increasesas the cell size scales down. In order to reduce the error rate, highanisotropy energy is required to overcome thermal fluctuations. Hardmagnetic material has higher anisotropy energy compared with softmagnetic material, but in that case a higher writing current isrequired. The higher anisotropy energy results in higher writing currentdensity, which unfortunately has the disadvantage of electro-migration.

In order to reduce the writing current for a high coercitivity MRAM,thermally assisted MRAMs are disclosed. Un-pinned ferromagneticmaterials, in which the coercitivity decreases sharply as temperatureincreases, are used for the recording layer in the MRAMs.

Another type of MRAM is spin-transfer torque memory (STRAM). STRAMutilizes electron spin torque to switch the free layer by passing a spinpolarized current thorough the STRAM. STRAM has a higher efficiency asthe memory cells scale down, but still suffers from the same issues asother MRAM cells as STRAM scales down. STRAM can also utilize thermalassist concept to reduce the switching current and maintain dataretention time.

However, thermally assisted MRAM suffer from low heating efficiency. Inaddition, due to Joule heating, heat gradually builds in the memoryarray structure which increases the temperature of the memory deviceduring operation.

BRIEF SUMMARY

The present disclosure relates to thermally assisted MRAM that includesphonon glass electron crystal material. In particular, the presentdisclosure relates to thermally assisted MRAM that utilize phonon glasselectron crystal material to confine heat within the MRAM. The presentdisclosure relates to thermally assisted MRAM that utilize two differentmaterials that generate a Peltier effect that assists in heating andcooling the MRAM cell.

One illustrative magnetic memory unit includes a tunneling barrierseparating a free magnetic element and a reference magnetic element. Afirst phonon glass electron crystal layer is disposed on a side opposingthe tunneling barrier of either the free magnetic element or thereference magnetic element.

One illustrative method includes applying a first current through amagnetic memory unit in a first direction. The magnetic memory unitincludes a magnetic tunnel junction separating a first phonon glasselectron crystal layer and a second phonon glass electron crystal layer.The first current causing a first interface between the magnetic tunneljunction and the first phonon glass electron crystal layer and a secondinterface between the magnetic tunnel junction and the second phononglass electron crystal layer to generate heat. The method then includesapplying a second current through the magnetic memory unit in a seconddirection opposing the first direction. The second current causing thefirst interface and the second interface to absorb heat.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1 is a schematic side view of an exemplary MRAM memory unit thatutilizes external magnetic fields;

FIG. 2 is a schematic side view of another exemplary MRAM memory unitthat utilizes spin-torque transfer;

FIG. 3 is a graph of current verses time for an exemplary MRAM memoryunit; and

FIG. 4 is a flow diagram of an illustrative method.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

In the following description, reference is made to the accompanying setof drawings that form a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.The definitions provided herein are to facilitate understanding ofcertain terms used frequently herein and are not meant to limit thescope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

The recitation of numerical ranges by endpoints includes all numberssubsumed within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, and 5) and any range within that range.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

The present disclosure relates to thermally assisted MRAM that includesphonon glass electron crystal material. In particular, the presentdisclosure relates to thermally assisted MRAM that utilize phonon glasselectron crystal material to confine heat within the MRAM. The presentdisclosure relates to thermally assisted MRAM that utilize two differentmaterials that generate a Peltier effect that assists in heating andcooling the MRAM cell While the present disclosure is not so limited, anappreciation of various aspects of the disclosure will be gained througha discussion of the examples provided below.

FIG. 1 is a schematic side view of an exemplary magnetic memory (MRAM)unit 10 and FIG. 2 is a schematic side view of an exemplary MRAM unit30. The MRAM memory unit 10 includes a reference magnetic element 20, afree magnetic element 30 and a tunneling barrier 25 separating thereference magnetic element 20 from the free magnetic element 30. Theseelements or layers are disposed electrically between a first electrode12 and a second electrode 14 forming a magnetic tunnel junction 26.While a single MRAM memory unit 10 is shown, it is understood that aplurality of MRAM memory units 10 can be arranged in an array to form amemory array.

An access transistor 16 is electrically connected to the first electrode12. The access or select transistor 16 can be electrically coupled to asourceline SL and a wordline WL, for example. The access transistor 16can be any useful transistor such as, for example, a NMOS or PMOSdevice. The access transistor 16 can provide a reading current, acooling current, and a heating current to the magnetic tunnel junction26.

The reference magnetic element 20 can include, at least, a referenceferromagnetic layer 21 and a reference antiferromagnetic layer 22. Thereference antiferromagnetic layer 22 serves to fix the magnetization ofthe reference ferromagnetic layer 21. The reference magnetic element 20has a blocking temperature (i.e., critical temperature) that issubstantially the Curie temperature of the reference magnetic element20. The blocking temperature of the reference antiferromagnetic layer 22is the temperature at or above which the reference antiferromagneticlayer 22 loses its ability to pin (i.e., fix) the magnetizationorientation of the adjacent reference ferromagnetic layer 21. In manyembodiments, the blocking temperature is above 250 degrees centigrade orin a range from 200 to 350 degrees centigrade. In some embodiments, thereference magnetic element 20 includes more than one ferromagnetic layerthat are coupled anti-ferromagnetically to each other (e.g., syntheticantiferromagnet). The synthetic antiferromagnet structure of thereference magnetic element 20 can reduce stray magnetic fieldinteractions with the free magnetic element 30. The referenceferromagnetic layer 21 can be formed of any useful material such as, forexample, alloys and materials including Co, Fe, and/or Ni. The referenceantiferromagnetic layer 22 can be formed of any useful material such as,for example, IrMn, FeMn, and/or PtMn.

The free magnetic element 30 can include, at least, a free ferromagneticlayer 31 and a free antiferromagnetic layer 32. The freeantiferromagnetic layer 32 serves to fix the magnetization of the freeferromagnetic layer 31. The free magnetic element 30 has a blockingtemperature (i.e., critical temperature) that is substantially the Curietemperature of the free magnetic element 30. The blocking temperature ofthe free antiferromagnetic layer 32 is the temperature at or above whichthe free antiferromagnetic layer 32 loses its ability to pin (i.e., fix)the magnetization orientation of the adjacent free ferromagnetic layer31. In many embodiments, the blocking temperature of the freeantiferromagnetic layer 32 is less than the blocking temperature of thereference antiferromagnetic layer 22. In many embodiments, the blockingtemperature of the free antiferromagnetic layer 32 is less than 150degrees centigrade or in a range from 50 to 150 degrees centigrade. Insome embodiments, the free magnetic element 30 includes more than oneferromagnetic layer that are coupled anti-ferromagnetically to eachother (e.g., synthetic antiferromagnet). The synthetic antiferromagnetstructure of the free magnetic element 30 can reduce stray magneticfield interactions with the reference magnetic element 20. The freeferromagnetic layer 31 can be formed of any useful material such as, forexample, alloys and materials including Co, Fe, and/or Ni. The freeantiferromagnetic layer 32 can be formed of any useful material such as,for example, IrMn, FeMn, and/or PtMn.

The tunneling barrier 25 separates the free magnetic element 30 from thereference magnetic element 20. The tunneling barrier 25 is anelectrically insulating and non-magnetic material. The tunneling barrier25 can be formed of any useful electrically insulating and non-magneticmaterial such as, AlO, MgO, and/or TiO, for example.

The magnetic memory unit 10 includes phonon glass electron crystallayers 41, 42 disposed adjacent to the free magnetic element 30 or thereference magnetic element 20. In the illustrated embodiment, a firstphonon glass electron crystal layer 41 is adjacent to free magneticelement 30 and a second phonon glass electron crystal layer 42 isadjacent to the reference magnetic element 20. In many embodiments, thefirst phonon glass electron crystal layer 41 separates the free magneticelement 30 from the second electrode 14 and the second phonon glasselectron crystal layer 42 separates the reference magnetic element 20from the reference magnetic element 20.

The phonon glass electron crystal material forming the layer 41, 42 iselectrically conducting but thermally insulating. In other words, phononglass electron crystal material possesses good electronic transportproperties of a crystal and resists the passage of heat as well as glassdoes. For example, the phonon glass electron crystal material formingthe layer 41, 42 has a thermal conductivity of less than 10 W/m-K and anelectrical area resistance of less than 100 Ohm/μm². Thus, the phononglass electron crystal material reduces or prevent heat from passingthrough the phonon glass electron crystal material. This can increaseenergy efficiency when heating the magnetic memory cell for thermallyassisted writing operations.

The phonon glass electron crystal layer can be formed of any usefulmaterial possessing the electrically conducting but thermally insulatingproperties described above. In many embodiments, the phonon glasselectron crystal layer is a superlattice structure formed of an alloy ofBiTeSe, CoAs, CeCoFeSb, SiGeC/Si, Bi₂Te₃/Sb₂Te₃ or X₈Y₁₆Z₃₀ where X isBa, Sr, or Eu; Y is Al, Ga or In; and Y is Si, Ge or Sn.

In many embodiments, the phonon glass electron crystal layer provides aPeltier effect on the magnetic tunnel junction when a current flowsthrough the memory unit. When a current passes through the memory unithaving an n type phonon glass electron crystal layer and an opposing ptype phonon glass electron crystal layer sandwiching the magnetic tunneljunction, the current causes the interfaces between the magnetic tunneljunction and the phonon glass electron crystal layers to either generateheat or absorb heat, depending on the direction of the current.

For example, when applying a first current through a magnetic memoryunit in a first direction, where the magnetic memory unit includes amagnetic tunnel junction separating a first phonon glass electroncrystal layer and a second phonon glass electron crystal layer, thefirst current causes an interface between the magnetic tunnel junctionand the first phonon glass electron crystal layer and the second phononglass electron crystal layer to generate heat. Electrons in an n typephonon glass electron crystal layer will move opposite the direction ofthe current and holes in the p type phonon glass electron crystal layerwill move in the direction of the current, both removing heat from oneside of the magnetic tunnel junction.

Likewise, when applying a second current through the magnetic memoryunit in a second direction opposing the first direction, the secondcurrent causes an interface between the magnetic tunnel junction and thefirst phonon glass electron crystal layer and the second phonon glasselectron crystal layer to absorb heat. P type silicon has a positivePeltier coefficient and n type silicon has a negative Peltiercoefficient.

N type (n for negative) dopants for semiconductor substrates (e.g.,phonon glass electron crystal material) include, phosphorus (P), arsenic(As), or antimony (Sb), boron (B) or aluminum (Al), for example. P type(P for positive) dopants are a certain type of atoms added to thesemiconductor substrates (e.g., phonon glass electron crystal material)in order to increase the number of free charge carriers (in this casepositive).

When the doping material is added to the semiconductor substrates (e.g.,phonon glass electron crystal material), it takes away (accepts)weakly-bound outer electrons from the semiconductor atoms. This type ofdoping agent is also known as acceptor material and the semiconductoratoms that have lost an electron are known as holes.

The purpose of p type doping is to create an abundance of holes. In thecase of silicon, a trivalent atom (often from group IIIA of the periodictable, such as boron or aluminum) is substituted into the crystallattice. The result is that one electron is missing from one of the fourcovalent bonds normal for the silicon lattice. Thus the dopant atom canaccept an electron from a neighboring atoms' covalent bond to completethe fourth bond. Such dopants are called acceptors. The dopant atomaccepts an electron, causing the loss of half of one bond from theneighboring atom and resulting in the formation of a “hole”. Each holeis associated with a nearby negative-charged dopant ion, and thesemiconductor remains electrically neutral as a whole. However, onceeach hole has wandered away into the lattice, one proton in the atom atthe hole's location will be “exposed” and no longer cancelled by anelectron. For this reason a hole behaves as a quantity of positivecharge. When a sufficiently large number of acceptor atoms are added,the holes greatly outnumber the thermally-excited electrons. Thus, theholes are the majority carriers, while electrons are the minoritycarriers in p type materials.

An n type semiconductor (including phonon glass electron crystalmaterial) is obtained by carrying out a process of doping, that is, byadding an impurity of valence-five elements to a valence-foursemiconductor in order to increase the number of free charge carriers(in this case negative). When the doping material is added, it givesaway (donates) weakly-bound outer electrons to the semiconductor atoms.This type of doping agent is also known as donor material since it givesaway some of its electrons.

The purpose of n type doping is to produce an abundance of mobile or“carrier” electrons in the material. To help understand how n typedoping is accomplished, consider the case of silicon (Si). Si atoms havefour valence electrons, each of which is covalently bonded with each ofthe four adjacent Si atoms. If an atom with five valence electrons, suchas those from group 15 (old group VA, a.k.a. nitrogen group) of theperiodic table (e.g., phosphorus (P), arsenic (As), or antimony (Sb)),is incorporated into the crystal lattice in place of a Si atom, thenthat atom will have four covalent bonds and one unbonded electron. Thisextra electron is only weakly bound to the atom and can easily beexcited into the conduction band. At normal temperatures, virtually allsuch electrons are excited into the conduction band. Since excitation ofthese electrons does not result in the formation of a hole, the numberof electrons in such a material far exceeds the number of holes. In thiscase the electrons are the majority carriers and the holes are theminority carriers. Because the five-electron atoms have an extraelectron to “donate”, they are called donor atoms. Note that eachmovable electron within the semiconductor (e.g., phonon glass electroncrystal material) is not far from an immobile positive dopant ion, andthe n doped material normally has a net electric charge of zero. In an ntype semiconductor, the fermi level lies closer to the conduction bandedge.

The magnetic memory cell 10, 30 is in the low resistance state when themagnetization orientation of the free magnetic layer 31 is parallel andin the same direction of the magnetization orientation of the referencemagnetic layer 21. This is termed the low resistance state or “0” datastate. The magnetic memory cell 10, 30 is in the high resistance statewhen the magnetization orientation of the free magnetic layer 31 isanti-parallel and in the opposite direction of the magnetizationorientation of the reference magnetic layer 21. This is termed the highresistance state or “1” data state.

FIG. 1 is a schematic side view of an exemplary MRAM memory unit 10 thatutilize external magnetic fields to switch the data state of the memoryunit. The free magnetic element 30 has a magnetization orientation thatis alterable or rotatable upon application of an external magnetic field(such as is produced by the first writing bit line WBL₁ and the secondwriting bit line WBL₂). A first writing bit line WBL₁ is electricallyisolated from the memory stack 26 and passes close enough to the memorystack 26 so that its magnetic field generated by a current 13 passingthought the first writing bit line WBL₁ can alter the magnetizationorientations of the recording magnetic layer 20 and/or the free magneticlayer 30. The first writing bit line WBL₁ longitudinally extends in afirst direction.

A second writing bit line WBL₂ is electrically isolated from the memorystack 26 and passes close enough to the memory stack 26 so that itsmagnetic field generated by a current 11 passing thought the secondwriting bit line WBL₂ can alter the magnetization orientations of therecording magnetic layer 20 and the free magnetic layer 30. The secondwriting bit line WBL₂ longitudinally extends in a second direction andin many embodiments is orthogonal to the first direction.

FIG. 2 is a schematic side view of another exemplary MRAM memory unit 30that utilize spin-torque transfer to switch the data state of the memoryunit. Switching the resistance state (between a high and low resistancestate) and hence the data state of the memory unit 30 via spin-transferoccurs when a current, passing through a magnetic layer of the memoryunit 30, becomes spin polarized and imparts a spin torque on the freemagnetic element 30 of the magnetic tunnel junction 26. When asufficient spin torque is applied to the free magnetic element 30, themagnetization orientation of the free magnetic element 30 can beswitched between two opposite directions and accordingly the magneticmemory cell 30 can be switched between the parallel state (i.e., lowresistance state or “0” data state) and anti-parallel state (i.e., highresistance state or “1” data state) depending on the direction of thecurrent.

FIG. 3 is a graph of current verses time for an exemplary MRAM memoryunit. For example, a positive current flowing through the magneticmemory units, described above heats the magnetic tunnel junction 26 anda negative current flowing through the magnetic memory units, describedabove cools the magnetic tunnel junction 26. Thus, these magnetic memoryconfigurations can be utilized with thermally assisted magnetic memorywhere the magnetic memory is heated to a temperature that reduces theswitching field or current needed to switch the memory unit between thehigh and low resistance state. Then passing a current in the opposingdirection removes heat from the magnetic tunnel junction and provides anexpedited cool down of the magnetic tunnel junction.

FIG. 4 is a flow diagram of an illustrative method 100. The methodincludes applying a first current I1 in a first direction through amagnetic tunnel junction (MTJ) having a phonon glass electron crystallayer (PGEC) to generate heat at block 101 The first current can besufficient to raise the temperature of a antiferromagnetic layeradjacent to the free magnetic layer above it blocking temperature. Thisreduces the electric field needed to switch the magnetizationorientation of the free magnetic layer at block 102. The phonon glasselectron crystal layer also improves the heating efficiency for the MTJby confining the generated heat to the MTJ. Then the MTJ can be cooleddown by applying a second current I2 in a second direction (opposing thefirst direction) through a magnetic tunnel junction (MTJ) to absorb heatat block 103. This active cool down increases the heat removal from theMTJ (since the phonon glass electron crystal layer reduces or preventsheat flow through the phonon glass electron crystal layer) reducing thewrite time and allows the read operation to proceed at block 104 whenthe MTJ is cooled down sufficiently.

Thus, embodiments of the MAGNETIC MEMORY WITH PHONON GLASS ELECTRONCRYSTAL MATERIAL are disclosed. The implementations described above andother implementations are within the scope of the following claims. Oneskilled in the art will appreciate that the present disclosure can bepracticed with embodiments other than those disclosed. The disclosedembodiments are presented for purposes of illustration and notlimitation, and the present invention is limited only by the claims thatfollow.

1. A magnetic memory unit, comprising: a tunneling barrier separating afree magnetic element and a reference magnetic element forming amagnetic tunnel junction; and a first phonon glass electron crystallayer disposed on a side opposing the tunneling barrier of either thefree magnetic element or the reference magnetic element.
 2. A magneticmemory unit according to claim 1, further comprising a second phononglass electron crystal layer, wherein the first phonon glass electroncrystal layer is disposed on the side opposing the tunneling barrier ofthe free magnetic layer, and the second phonon glass electron crystallayer is disposed on the side opposing the tunneling barrier of thereference magnetic layer.
 3. A magnetic memory unit according to claim1, wherein the phonon glass electron crystal layer has a thermalconductivity of less than 10 W/mK and an electrical area resistance ofless than 100 Ohm/μm².
 4. A magnetic memory unit according to claim 1,wherein the magnetic tunnel junction switches between a high resistancedata state and a low resistance data state by passing a spin-polarizedcurrent though the magnetic tunnel junction.
 5. A magnetic memory unitaccording to claim 1, wherein the phonon glass electron crystal layercomprises an alloy of BiTeSe, CoAs, CeCoFeSb, SiGeC/Si, Bi₂Te₃/Sb₂Te₃ orX₈Y₁₆Z₃₀ where X is Ba, Sr, or Eu, Y is Al, Ga or In, and Y is Si, Ge orSn.
 6. A magnetic memory unit according to claim 1, wherein thereference magnetic element comprises a reference ferromagnetic layer anda reference antiferromagnetic layer having a reference blockingtemperature, and the free magnetic element comprises a freeferromagnetic layer and a free antiferromagnetic layer having a freeblocking temperature being less than the reference blocking temperature.7. A magnetic memory unit according to claim 2, wherein the phonon glasselectron crystal layers provide a Peltier effect on the magnetic tunneljunction when current flows through the magnetic memory unit.
 8. Amemory unit, comprising: a tunneling barrier separating a free magneticelement and a reference magnetic element forming a magnetic tunneljunction; a first phonon glass electron crystal layer disposed adjacentto the free magnetic element; a second phonon glass electron crystallayer disposed adjacent to the reference magnetic element, wherein thefirst phonon glass electron crystal layer and the second phonon glasselectron crystal layer are formed of different materials; and a firstelectrode is electrically coupled to the first phonon glass electroncrystal layer and a second electrode is electrically coupled to thesecond phonon glass electron crystal layer.
 9. A memory unit accordingto claim 8, wherein the first and second phonon glass electron crystallayers have a thermal conductivity of less than 10 W/mK and anelectrical area resistance of less than 100 Ohm/μm².
 10. A memory unitaccording to claim 8, wherein the magnetic tunnel junction switchesbetween a high resistance data state and a low resistance data state bypassing a spin-polarized current though the magnetic tunnel junction.11. A memory unit according to claim 8, wherein the magnetic tunneljunction switches between a high resistance data state and a lowresistance data state by applying an external magnetic field through themagnetic tunnel junction.
 12. A memory unit according to claim 8,wherein the first and second phonon glass electron crystal layers eachindependently comprises BiTeSe, CoAs, CeCoFeSb, SiGeC/Si, Bi₂Te₃/Sb₂Te₃or X₈Y₁₆Z₃₀ where X is Ba, Sr, or Eu, Y is Al, Ga or In, and Y is Si, Geor Sn.
 13. A memory unit according to claim 8, wherein the referencemagnetic element comprises a reference ferromagnetic layer and areference antiferromagnetic layer having a reference blockingtemperature, and the free magnetic element comprises a freeferromagnetic layer and a free antiferromagnetic layer having a freeblocking temperature being less than the reference blocking temperature.14. A memory unit according to claim 8, wherein the first phonon glasselectron crystal layer is formed of a p type semiconductor material andthe second phonon glass electron crystal layer is formed of an n typesemiconductor material.
 15. A memory unit according to claim 8, whereinthe first phonon glass electron crystal layer is formed of an n typesemiconductor material and the second phonon glass electron crystallayer is formed of a p type semiconductor material.
 16. A method,comprising: applying a first current through a magnetic memory unit in afirst direction, the magnetic memory unit comprising a magnetic tunneljunction separating a first phonon glass electron crystal layer and asecond phonon glass electron crystal layer, the first current causing afirst interface between the magnetic tunnel junction and the firstphonon glass electron crystal layer and a second interface between themagnetic tunnel junction and the second phonon glass electron crystallayer to generate heat; and applying a second current through themagnetic memory unit in a second direction opposing the first direction,the second current causing the first interface and the second interfaceto absorb heat.
 17. A method according to claim 16, wherein the firstand second phonon glass electron crystal layers have a thermalconductivity of less than 10 W/mK and an electrical area resistance ofless than 100 Ohm/μm².
 18. A method according to claim 16, wherein theapplying a first current step comprises heating the magnetic tunneljunction above a blocking temperature of an antiferromagnetic layeradjacent to a free magnetic layer of the magnetic tunnel junction.
 19. Amethod according to claim 18, further comprising switching a magneticorientation of the free magnetic layer before applying the secondcurrent through the magnetic memory unit.
 20. A method according toclaim 16, further comprising passing a read current through the magnetictunnel junction to read a data state of the magnetic tunnel junction,after the applying a second current step.